Dual single-crystal backplate microphone system and method of fabricating same

ABSTRACT

A dual backplate MEMS microphone system includes a flexible diaphragm sandwiched between two single-crystal silicon backplates. Such a MEMS microphone system may be formed by fabricating each backplate in a separate wafer, and then transferring one backplate from its wafer to the other wafer, to form two separate capacitors with the diaphragm.

PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 12/870,288, filed on Aug. 27, 2010, by Kuang L. Yang, andentitled “Method Of Fabricating A Dual Single-Crystal BackplateMicrophone”, which claims priority from provisional U.S. patentapplication Ser. No. 61/238,014, filed Aug. 28, 2009, entitled, “DualSingle-crystal Backplate Microphone System and Method of FabricatingSame,” and naming Kuang L. Yang and Li Chen as inventors, the disclosureof which is incorporated herein, in its entirety, by reference.

TECHNICAL FIELD

The present invention relates to micromachined microphones, and moreparticularly to methods of fabrication of micromachined microphones.

BACKGROUND ART

Microphones generally have a movable diaphragm arranged parallel to astationary backplate. The diaphragm and backplate form a variablecapacitor. The diaphragm moves in response to incident acoustic energyto change the variable capacitance and thereby produce an electricalsignal representative of the incident acoustic energy.

Polysilicon can be micromachined to fabricate both the diaphragm and/orbackplate. However, polysilicon micromachined structures may retainstress from their fabrication, and may deform, buckle or even break whencooled. Polysilicon microelectromechanical system (“MEMS”) structuresmay also have surfaces that are not generally planar (i.e., they may beirregularly wavy), which may potentially adversely effect theirqualities as the plate of a variable capacitor.

SUMMARY OF THE ILLUSTRATIVE EMBODIMENTS

In a first embodiment, a dual-backplate micromachined microphone has adiaphragm movably sandwiched between two single-crystal backplates toform two variable capacitors—one with each backplate. Two insulativelayers, such as oxide layers, electrically isolate the backplates fromthe diaphragm.

Some embodiments have various contacts in one or both of the backplatelayers, to facilitate electrical connections to the diaphragm and one orboth of the backplates. Illustrative embodiments include a backsidecavity adjacent to one of the backplate layers, to provide physicalsupport for the microphone, and to provide a path for acoustic energy toreach the diaphragm. In some embodiments, one or both of the backplatesare part of a silicon-on-insulator wafer, while the diaphragm may bepolysilicon. In some embodiments, one or both backplate layers, or adiaphragm layer, include standoffs to regulate the gap between thebackplate and the diaphragm.

A dual-backplate microphone may be fabricated by providing a first waferwith a first conductive single-crystal backplate and a conductivediaphragm, and a second wafer with a second conductive single-crystalbackplate, and bonding the second backplate to the first wafer such thatthe diaphragm is sandwiched between, but electrically isolated from, thefirst backplate and the second backplate. The diaphragm may thus formcoupled capacitors, one with the first backplate and another with thesecond backplate.

One or both of the backplates may be in a layer of asilicon-on-insulator wafer. In some embodiments, one or both of thebackplates or the diaphragm layer may include standoffs. Someembodiments include a removable layer on one or both of the backplatesto contact and immobilize the diaphragm during fabrication. In someembodiments, removable layers are ultimately removed to release thediaphragm.

The assembly process may involve transferring the second backplate fromthe second wafer to the first wafer. In some embodiments, the secondwafer also includes a sacrificial (or adhesive) layer and a donorsubstrate, and the process includes removing the donor substrate andsacrificial layer after the wafers are bonded.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 schematically shows a mobile telephone that may use a MEMSmicrophone configured in accordance with illustrative embodiments of theinvention.

FIG. 2 schematically shows a MEMS microphone that may be configured inaccordance with illustrative embodiments of the invention.

FIG. 3 schematically shows a cross-sectional view of the microphoneshown in FIG. 2 across line 2-2.

FIG. 4 schematically shows a MEMS microphone system having dualsingle-crystal backplates according to one embodiment;

FIG. 5 shows a process of forming a dual-backplate microphone inaccordance with illustrative embodiments of the invention.

FIGS. 6A-6C schematically illustrate component parts of a microphoneaccording to various embodiments.

FIGS. 7A-7E schematically illustrate component parts of a microphoneaccording to various stages of fabrication in an illustrativeembodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a MEMS microphone is configured to have thedetection capability for high maximum sound pressure, broad bandwidth,and provide a decreased risk of pull-down. To those ends, adual-backplate microphone has a movable diaphragm sandwiched between twosingle-crystal backplates.

FIG. 1 schematically shows a mobile telephone 10 that can use amicrophone configured in accordance with illustrative embodiments. Insimplified terms, the telephone 10 has a receiver 12 for receiving anaudio signal (e.g., a person's voice), a speaker portion 14 forgenerating audio signals, and a transponder 16 for transmitting andreceiving electromagnetic signals encoding audio signals. During use, aperson may speak into the receiver 12, which has a MEMS microphone (FIG.2, discussed below) that converts the person's voice into an electricalsignal. Internal logic (not shown) and the transponder 16 modulate thissignal to a remote source, such as to a satellite tower and, ultimately,to another person on another telephone 10.

In illustrative embodiments, the receiver 12 has a microphonemechanically configured with a relatively precise low frequency cutoffpoint (i.e., the lowest frequency that it can detect without significantdistortion—often referred to in the art as the “−3 dB point”). FIG. 2schematically shows a top, perspective view of a portion of a MEMSmicrophone 18 (also referred to as a “microphone chip 18”). FIG. 3schematically shows a cross-sectional view of the same microphone 18across line 2-2 of FIG. 2. For clarity of illustration, neither FIG. 2nor FIG. 3 show a second backplate.

Among other things, the microphone 18 includes a static backplate 20that supports and forms a variable capacitor with a flexible diaphragm22. In illustrative embodiments, the backplate 20 is formed from singlecrystal silicon (e.g., the top layer of a silicon-on-insulator wafer,discussed below), while the diaphragm 22 is formed from a depositedmaterial, such as deposited polysilicon. Other embodiments, however, useother types of materials to form the backplate 20 and the diaphragm 22.For example, a single crystal silicon bulk wafer, or some depositedmaterial, may form the backplate 20. In a similar manner, a singlecrystal silicon bulk wafer, part of a silicon-on-insulator wafer, orsome other deposited material may form the diaphragm 22. To facilitateoperation, the backplate 20 has a plurality of through-hole apertures(“backplate apertures 24”) that lead to a backside cavity 26.

Springs 28 movably connect the diaphragm 22 to a static/stationaryportion 40 of the microphone 18, which includes a substrate (alsoidentified by reference number “30”). The springs 28 effectively form aplurality of apertures that permit at least a portion of the acousticenergy to pass through the diaphragm 22. These apertures 32, which alsoare referred to as “diaphragm apertures 32,” may be any reasonableshape, such as in the shape of a slot, round hole, or some irregularshape. Other embodiments, however, may have other types of springs 28and apertures 24 and 32.

Incident acoustic energy causes the diaphragm 22 to vibrate, thusproducing a changing capacitance between it and the backplate 20. Suchacoustic energy may contact the microphone 18 from any direction. Forexample, in FIG. 3, the acoustic energy is shown as traveling upwardly,first through the backplate 20, and then partially through and againstthe diaphragm 22. In other embodiments, the acoustic energy may travelin the opposite direction. On-chip or off-chip circuitry (not shown)receive (via contacts 36 of FIG. 2) and convert this changingcapacitance into electrical signals that can be further processed.

It should be noted that discussion of the specific microphone 18 shownin FIGS. 2-3 is for illustrative purposes only.

In accordance with illustrative embodiments, a dual-backplate microphone400 has a pair of generally parallel, single-crystal silicon backplatelayers 402 and 412, each with a backplate 401 and 411, respectively, anda polysilicon diaphragm layer 408 with a movable, conductive diaphragm409 sandwiched between the backplate layers 401, 411, as schematicallyillustrated in FIG. 4. The arrangement of the diaphragm 409 andbackplates 401 and 411 might be described as forming a stack.

Generally, a single-crystal backplate will be more rigid, and have amore consistent surface, than a polysilicon backplate. This is incontrast to prior art designs, which have a single backplate, and/or apolysilicon backplate.

The diaphragm 409 forms two capacitors—one with each backplate 401, 411.When incident acoustic energy impinges on the diaphragm 409, thediaphragm moves or vibrates. As the diaphragm moves closer to onebackplate, it moves further from the other backplate. Becausecapacitance is inversely proportional to the gap between the diaphragmand back-plate, the capacitance formed by the diaphragm moving towardsone back-plate increases, while the capacitance formed by the diaphragmmoving away from the other back-plate decreases.

When the diaphragm 409 is stationary, the distances between thediaphragm and each of the backplates 401, 411 are determined by otherstructures within the microphone 400. In FIG. 4, the gap 415 between thediaphragm 409 and the first backplate 401 is determined by thedimensions of bonding media 410, while the gap 414 between the diaphragm409 and the second backplate 411 is determined by the dimensions ofbonding media 413. The rigidity of a single-crystal backplate resistswarping of the backplate in response to external forces or impingingacoustic energy, and may thus allow a more consistent gap between thebackplates and the diaphragm than other backplates, such asmicromachined polysilicon backplates, for example.

Some embodiments include conductive connectors as part of the microphonestructure to facilitate electrical connections to the various featuresof the microphone 400. For example, contact 420 on second backplatelayer 412 is electrically coupled to the backplate layer 412 and to thesecond backplate 411. This allows the second backplate 411 to beelectrically connected to other circuits within a microphone system thatmay, for example, supply power to the backplate 411, or sense thecapacitance formed by the backplate 411 and the diaphragm 409.

Another contact 421 coupled to a via 422 passes through the secondbackplate layer 412 and electrically connects to the diaphragm 409. Thevia 422 is insulated from the second backplate layer 412 by insulatingoxide lining 423.

The first backplate 401 is electrically coupled to yet another contact424, through via 425 that passes through the second backplate layer 412.The via 425 is insulated from the second backplate layer 412 byinsulating oxide lining 426.

The microphone 400 of FIG. 4 also includes a backside cavity 404 throughthe bottom silicon (handle) layer 405 and intermediate oxide (e.g., theburied oxide, or “BOX”) layer 406 of the silicon-on-insulator (“SOI”)wafer 403. The backside cavity 404 provides an avenue by which incomingacoustic energy may reach the diaphragm 409, while intermediate oxide406 acts as an insulator that electrically separates the backplate 401and the handle layer 405.

The remains 430 of the handle layer 405 provide a structure to supportthe microphone 400 on a substrate. There is no corresponding structureon second backplate layer 412. Such a corresponding structure would makethe microphone 400 taller, and thus occupy more volume, which may beundesirable in some applications (for example, inside a cell phone or ICpackage, in which space is at a premium).

Fabricating a dual-backplate microphone with single-crystal backplatescannot be accomplished by growing a single-crystal backplate layer insitu adjacent to a diaphragm or another wafer, due to the nature ofsilicon crystal growth. However, such a microphone may be fabricated byproducing at least one of the single-crystal backplates separately, andfabricating the microphone by assembling its component parts by, forexample, wafer bonding.

In some embodiments, individually fabricating features on severalcomponent wafers (such as two SOI wafers, or an SOI wafer and asingle-crystal wafer, for example), and then bonding the waferstogether, may be performed prior to the complete fabrication of at leastone of the wafers (i.e., when some fabrication steps remain for at leastone of the wafers).

An illustrative embodiment of such a process is described in the flowchart of FIG. 5. Illustrative embodiments of certain component partsthat may be used in the process of FIG. 5 are schematically illustratedin FIGS. 6A, 6B and 6C, and illustrative embodiments of component partsbeing assembled according to the process of FIG. 5 are illustrated inFIGS. 7A-7E.

The process begins as step 501, in which a first backplate and diaphragmare provided on a first wafer. The first wafer may be an SOI wafer, andthe first backplate and the diaphragm may be fabricated in ways known inthe art.

For example, in FIG. 6A the first backplate 604 is schematicallyillustrated in the device layer (or backplate layer) 603 of SOI wafer600. The backplate 604 includes one or more holes/apertures/passages 605that may permit the passage of sound waves or acoustic energy to thediaphragm 606.

The first backplate 604 forms a capacitor with conductive diaphragm 606.To that end, an insulative oxide layer 608 physically and electricallyseparates the diaphragm layer 607 from the backplate layer 603. Thewafer 600 also includes a handle layer 601, and an intermediate oxidelayer 602, which include a backside cavity 611.

In some embodiments, the diaphragm 606 may be secured to the first wafer600 by a removable or sacrificial material 791, as schematicallyillustrated in FIG. 7D. The material 791 may serve to immobilize thediaphragm 606 during subsequent processing. Among other things, this mayreduce or eliminate the risk of the diaphragm 606 contacting thebackplate 604 during assembly of the dual-backplate microphone. Thematerial 791 may be a sacrificial polysilicon or oxide, for example.

A second backplate is provided on a second wafer (step 502). The secondwafer may be known as a “donor” wafer, and it has a “donor” portion thatis separable from the second backplate. A backplate illustratively isabout 10 microns thick and fabricated from single-crystal silicon.

For example, in FIG. 6B a second wafer 620 includes a second backplate624 in a device layer (which may also be known as the second backplatelayer) 623. The second backplate 624 includes one or more trenches 625that will later form holes, apertures or passages through the backplate624. The handle layer 621 is a donor layer (in other words, layer 621will ultimately donate backplate layer 623 to the dual-backplatemicrophone being fabricated).

An alternate embodiment of a second wafer is schematically illustratedin FIG. 6C. Wafer 630 includes a damage plane 631 where ions or otherimpurities have been implanted. In one embodiment, for example, ions(such as hydrogen ions) are implanted below the surface 632 of the wafer630 to form damage plane 631 parallel to the surface 632.

The top surface 632 includes trenches 635 to partially form backplate634. The trenches 635 will form apertures through the second backplate634 after the layer transfer is complete, as discussed below.

As such, the region of the wafer 630 above the damage plane 631 is thesecond backplate layer. The damage plane 631 and the portion 633 of thesecond wafer 630 below the damage plane (e.g., on the side of the secondwafer 630 opposite the trenches 635) is the donor layer. The donor layermay ultimately be separated from the backplate 624 by fracturing orsevering the wafer 630 at the damage plane 631.

In either case, the second backplate (624 or 634) may have a removableor sacrificial material 790, as schematically illustrated in FIG. 7D.The material 790 may serve to immobilize the diaphragm 606 duringsubsequent processing as schematically illustrated in FIG. 7E, and maybe a sacrificial polysilicon, polymer, or oxide, for example.

In some applications, it may be important that the gap between thediaphragm and the second backplate be substantially the same as the gapbetween the diaphragm and the first backplate. As such, in someembodiments, one of the second backplate layer or the diaphragm layer,or both, may include one or more non-conductive standoffs, such asstandoffs 610 in FIG. 6A, standoff 626 in FIG. 6B, and standoff 636 inFIG. 6C. Such standoffs stand in relief of the face of the surface towhich they are attached, and their height should match the gap betweenthe diaphragm and the first backplate. In this way, the standoffs willprevent the gap between the diaphragm and the second backplate frombeing smaller than desired (e.g., smaller than the gap between thediaphragm and the first backplate). In some embodiments, such a standoffmay also serve as a conductor between a contact on the second backplatelayer and another member of the microphone, such as the diaphragm or thefirst backplate.

Optionally, integrated circuits, such as transistors and other activedevices, passive devices, and interconnection structures, to name but afew, may be fabricated (step 503) on the second wafer. Fabricating asecond backplate or other features of the microphone structure, such asintegrated circuits, on the second wafer provides potential benefitsthat include lower cost of fabrication, and without risk of damage fromthe fabrication of micromachined structures (such as a diaphragm, forexample) on the same substrate. For example, such fabrication canperformed at temperatures lower than the dangerously high temperaturesat which some micromachined structures are fabricated.

The process then continues to step 504, in which the second wafer isplaced in contact with, and bonded to, the first wafer. FIG. 7A and FIG.7B schematically illustrative step 504 using certain component partsillustrated in FIGS. 6A and 6B. Although this illustrative embodimentemploys SOI wafers, it should be noted that various embodiments are notlimited to the use of SOI wafers. For example, single-crystal (non-SOI)wafers, such as the second wafer schematically illustrated in FIG. 6C,could also be used.

A first wafer 600 with a first backplate 604 and diaphragm 606 indiaphragm layer 607, and a second wafer 620 with a second backplatelayer 623, are provided and arranged so that the second backplate layer623 faces the diaphragm layer 607, as schematically illustrated in FIG.7A. If there are electrical interconnects or vias (such as thoseillustrated in FIG. 4, for example), the contact points on the secondwafer 620 should align with counterpart contacts on the first wafer 600(for example, the contacts may be on diaphragm layer 607).

One or more interconnect materials 701 are placed in-between the firstwafer 600 and second wafer 620. The interconnect materials 701 may beadhesive or other conductive or non-conductive bonding material. Theinterconnect material 701 may be initially attached to one of the firstwafer 600, the second wafer 620, or both. In some embodiments, theinterconnect materials 701 may initially be on the diaphragm layer 607and extend across the diaphragm 606 and between the diaphragm 606 andthe second backplate 624 to act as a sacrificial material 790, asdiscussed above, and illustrated in FIG. 7E.

In any case, the interconnect materials 701 are laterally spaced fromthe diaphragm 606 so as to avoid interfering with the movement of thediaphragm 606 in the finished product (for example, as in FIG. 7C).

The two wafers 600 and 620 and the interconnect materials 701 are thenbrought together and bonded to each other, to form a unified member asillustrated in FIG. 7B. As such, the two wafers are bonded via theinterconnect materials 701. In alternate embodiments, the bonding may bedone without inserting bonding materials, for example with metal bonds,or oxide direct bonding.

In some embodiments, the bond between the wafers is formed at atemperature below that at which a MEMS structure or integrated circuitsmay be damaged. In some embodiments, the bond is fabricated attemperatures of about 200 to 400 degrees Celsius, for example.

Once the wafers are bonded, portions (e.g., donor portions) of thesecond wafer are removed (step 505), leaving the second backplate bondedto the first wafer. For example, in FIG. 7C the donor layer 621 andintermediate oxide layer 622 of the second wafer 620 have been removed,leaving the second backplate layer 623 attached to the first wafer 600(and specifically in this embodiment, to diaphragm layer 607). As such,the former trenches 635 perforate (e.g., form holes or passages 702through) the second backplate layer 623 to form backplate 624.

Removing the donor substrate 621 may involve simply removing theintermediate oxide layer 622, thus allowing the donor substrate 621 tofall away. Alternately, the donor substrate may be removed by etching,grinding, or other methods known in the art.

Still other embodiments may remove portions of the second wafer by acombination of etching, and grinding or lapping down the portions to beremoved. For example, if the wafer is an SOI wafer (620), the handlelayer (621) may be removed by grinding or lapping down, to expose theinsulator layer (622), which may then be removed by etching.

In any case, such processes are preferably low temperature process thatwill not damage MEMS structures or circuits (such as CMOS circuits) inthe device.

Some processes leave the donor wafer intact, while others leave itintact but thinner than it was originally. The donor wafer may be reusedif it remains thick enough to have a transferable layer fabricated onits face. For example, in some embodiments, a new backplate 624 may beformed on the donor wafer 620 by fabricating a sacrificial layer 622(such as an oxide layer which may act as an adhesive layer) on the donorwafer 620, and then fabricating or bonding the new backplate 624 ontothe sacrificial layer. In some embodiments, the substrate (donor layer)need not be a wafer, per se, but could be any substrate that is capableof supporting the fabrication of a sacrificial layer and backplatelayer. As such, illustrative embodiments may refer to a second “wafer,”but it is understood that the structure is not limited to wafers orsemiconductor wafers.

Other post-processing (step 506) may be performed before the processends, such as polishing surfaces (e.g., through CMP or a mechanicalgrind) or interconnecting circuits, for example. At some point, thediaphragm may be released (step 507)—for example, by removing at leastsome of the removable or sacrificial material (790 and/or 791, if any)or other remaining structure between the diaphragm 606 and one or bothof the backplates 604, 624. Some embodiments that include suchsacrificial materials may have only one of the sacrificial materials 790or 791.

Thus, generally speaking, some embodiments of a dual backplatemicrophone have a first single-crystal backplate, a secondsingle-crystal backplate, and a flexible diaphragm positioned (orsandwiched) between the first and second backplates. The backplates anddiaphragm form a stack, and are separated by a small gap. The diaphragmforms two variable capacitors—one with each of the two backplates.Electrical connections to the diaphragm and each backplate allow eachcapacitance to be sensed by connected circuitry. The backplates arepreferably substantially rigid and will not deform in response to anyincident acoustic energy.

A dual-backplate microphone may provide a number of advantages oversingle-backplate microphones, due, for example, to certain physicalproperties of the single-crystal backplate, such as its rigidity andsmooth surface.

Advantages of a dual-backplate microphone system may include widedynamic range, including relatively high maximum sound pressure (e.g.,greater than 120 dB spl; in some embodiments up to 160 dB spl orgreater), and broad bandwidth (e.g., greater than 20 kHz; in someembodiments up to 100 kHz or greater), to name but a few. Also, with adifferential capacitor, bias voltages may be employed that are higherthan those used in single-backplate designs. The electrostatic forcethat may attract the diaphragm to the backplate on one side of thediaphragm is, to some degree, counterbalanced by an electrostatic forcebetween the diaphragm and the backplate on the opposite side. Thesecompeting, offsetting forces reduce the likelihood that the diaphragmwill be pulled to one backplate (a phenomenon that may be known as“pulldown”) and irretrievably stuck, and thus, higher bias voltages maybe applied.

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inany appended claims.

What is claimed is:
 1. A dual-backplate microphone comprising: a firstbackplate layer comprising a first conductive, single-crystal backplate;a second backplate layer comprising a second conductive, single-crystalbackplate; a conductive diaphragm movably sandwiched between andelectrically isolated from the first and second backplates, wherein thebackplates and the conductive diaphragm form a stack such that theconductive diaphragm forms a first variable capacitor with the firstbackplate and a second variable capacitor with the second backplate; afirst oxide layer physically and electrically separating the conductivediaphragm from the first backplate layer; a second oxide layerphysically and electrically separating the conductive diaphragm from thesecond backplate layer; a first contact coupled to a first via, thefirst via configured to pass through the second backplate andelectrically connect to the diaphragm, the first via being insulatedfrom the second backplate by the first oxide layer; and a second contactcoupled to a second via, the second via configured to pass through thesecond backplate and electrically connect to the first backplate, thesecond via being insulated from the second backplate by the second oxidelayer.
 2. A dual-backplate microphone according to claim 1, wherein thesecond backplate layer further comprising: a second backplate contactelectrically coupled to the second backplate; and a diaphragm contactelectrically coupled to the conductive diaphragm.
 3. A dual-backplatemicrophone according to claim 2, wherein the second backplate layerfurther comprising a first backplate contact electrically coupled to thefirst backplate.
 4. A dual-backplate microphone according to claim 1,wherein the first backplate layer further comprising a first side and asecond side, the first side facing the diaphragm, and the second sidecomprising a backside cavity.
 5. A dual-backplate microphone accordingto claim 1, wherein the second backplate layer further comprises a firstside and a second side, the first side facing the conductive diaphragm,and the second side comprises a backside cavity.
 6. A dual-backplatemicrophone according to claim 1, wherein at least one of the firstbackplate layer and the second backplate layer comprises an SOI wafer.7. A dual-backplate microphone according to claim 1, wherein theconductive diaphragm, comprises polysilicon.
 8. A dual-backplatemicrophone according to claim 1, further comprising one or morestandoffs between the second backplate layer and the conductivediaphragm layer.
 9. A dual-backplate microphone according to claim 8,wherein the standoffs stand in relief from the second backplate layer.10. A dual-backplate microphone according to claim 8, wherein thestandoffs stand in relief from the conductive diaphragm.
 11. Adual-backplate microphone according to claim 1, further including a pairof springs individually situated on either side of the conductivediaphragm to moveably connect the conductive diaphragm to a stationaryportion of the microphone.